lib/commit: respect SOURCE_DATE_EPOCH for commit timestamp

This tweaks `ostree_repo_write_commit` so that it checks for the
envinroment variable `SOURCE_DATE_EPOCH` as a way to override
the current time, which is used as the commit timestamp.

Ref: https://reproducible-builds.org/docs/source-date-epoch/
Ref: https://reproducible-builds.org/specs/source-date-epoch/

Makefile-tests.am

@@ -69,6 +69,7 @@ _installed_or_uninstalled_test_scripts = \
 	tests/test-remote-add.sh \
 	tests/test-remote-headers.sh \
 	tests/test-commit-sign.sh \
+	tests/test-commit-timestamp.sh \
 	tests/test-export.sh \
 	tests/test-help.sh \
 	tests/test-libarchive.sh \

src/libostree/ostree-repo-commit.c

@@ -3020,6 +3020,10 @@ create_empty_gvariant_dict (void)
  *
  * Write a commit metadata object, referencing @root_contents_checksum
  * and @root_metadata_checksum.
+ * This uses the current time as the commit timestamp, but it can be
+ * overridden with an explicit timestamp via the
+ * [standard](https://reproducible-builds.org/specs/source-date-epoch/)
+ * `SOURCE_DATE_EPOCH` environment flag.
  */
 gboolean
 ostree_repo_write_commit (OstreeRepo      *self,
@@ -3032,9 +3036,26 @@ ostree_repo_write_commit (OstreeRepo      *self,
                           GCancellable    *cancellable,
                           GError         **error)
 {
-  g_autoptr(GDateTime) now = g_date_time_new_now_utc ();
+  gint64 timestamp = 0;
+  const gchar *env_timestamp = g_getenv ("SOURCE_DATE_EPOCH");
+  if (env_timestamp == NULL)
+    {
+      g_autoptr(GDateTime) now = g_date_time_new_now_utc ();
+      timestamp = g_date_time_to_unix (now);
+    }
+  else
+    {
+      gchar *ret = NULL;
+      errno = 0;
+      timestamp = g_ascii_strtoll (env_timestamp, &ret, 10);
+      if (errno != 0)
+        return glnx_throw_errno_prefix (error, "Parsing SOURCE_DATE_EPOCH");
+      if (ret == env_timestamp)
+        return glnx_throw (error, "Failed to convert SOURCE_DATE_EPOCH");
+    }
+
   return ostree_repo_write_commit_with_time (self, parent, subject, body,
-                                             metadata, root, g_date_time_to_unix (now),
+                                             metadata, root, timestamp,
                                              out_commit, cancellable, error);
 }
 

tests/test-commit-timestamp.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+#
+# SPDX-License-Identifier: LGPL-2.0+
+
+set -euo pipefail
+
+. $(dirname $0)/libtest.sh
+TZ='UTC'
+LANG='C'
+
+echo "1..2"
+
+# Explicit timestamp via CLI flag.
+mkdir testrepo
+ostree_repo_init testrepo --mode="archive"
+mkdir testrepo-files
+cd testrepo-files
+echo first > firstfile
+cd ..
+${CMD_PREFIX} ostree --repo=./testrepo commit -b cli --timestamp='@1234567890' -s "cli timestamp"
+${CMD_PREFIX} ostree --repo=./testrepo show cli > show-cli.txt
+rm -rf testrepo testrepo-files
+assert_file_has_content_literal show-cli.txt 'Date:  2009-02-13 23:31:30 +0000'
+echo "ok commit with CLI timestamp"
+
+# Reproducible timestamp via env flag.
+mkdir testrepo
+ostree_repo_init testrepo --mode="archive"
+mkdir testrepo-files
+cd testrepo-files
+echo first > firstfile
+cd ..
+${CMD_PREFIX} SOURCE_DATE_EPOCH='1234567890' ostree --repo=./testrepo commit -b env -s "env timestamp"
+if (${CMD_PREFIX} SOURCE_DATE_EPOCH='invalid' ostree --repo=./testrepo commit -b env -s "invalid timestamp") 2> commit-invalid.txt; then
+    assert_not_reached "commit with invalid timestamp succeeded"
+fi
+if (${CMD_PREFIX} SOURCE_DATE_EPOCH='12345678901234567890' ostree --repo=./testrepo commit -b env -s "overflowing timestamp") 2> commit-overflowing.txt; then
+    assert_not_reached "commit with overflowing timestamp succeeded"
+fi
+${CMD_PREFIX} ostree --repo=./testrepo show env > show-env.txt
+rm -rf testrepo testrepo-files
+assert_file_has_content_literal commit-invalid.txt 'Failed to convert SOURCE_DATE_EPOCH'
+assert_file_has_content_literal commit-overflowing.txt 'Parsing SOURCE_DATE_EPOCH: Numerical result out of range'
+assert_file_has_content_literal show-env.txt 'Date:  2009-02-13 23:31:30 +0000'
+echo "ok commit with env timestamp"